In recent year, attention has been given to a new technique called imaging mass spectrometry (hereinafter, referred to as “IMS”) for analyzing an organism at the molecular level and displaying the analysis as an image in the fields of biochemistry and medicine. IMS is a method in which an arbitrary region of a sample is ionized using, for example, secondary ion mass spectrometry (hereinafter, referred to as “SIMS”), laser desorption/ionization (hereinafter, referred to as “LDI”), or matrix-assisted laser desorption/ionization (hereinafter, referred to as “MALDI”), followed by mass spectrometry using time-of-flight mass spectrometry (TOFMS), whereby a material distribution and a localized state of the sample is visualized (Non-Patent Documents 1 and 2). When this technique is used for the measurement of various organic compounds such as protein, peptide, and endocrine disrupting chemicals, a functional change can be detected at the cellular level, for example, enabling very early diagnosis, tailor-made medicine, the selection of candidates for drug development, investigating the delivery of developed drugs, the elucidation of a vital phenomenon and a disease, and the like. Thus, the technique is expected to be extremely useful.
More specifically, IMS using SIMS is a method in which a sample is irradiated with a primary ion beam accelerated and converged to 3 to 25 keV in high vacuum, so that secondary ions generated when materials are sputtered from a surface of the sample are utilized. In general, a liquid metal ion source (hereinafter, referred to as an “LMI”) that generates an ion beam of G+ or In+ is used as a primary ion source, and the diameter of a converged ion beam is generally 1 μm and could be up to 100 nm. A Cs+ ion gun is an inexpensive primary ion source that realizes a spot diameter of 2 to 3 μm.
Further, LDI is a method that uses a laser beam instead of a primary beam as used in SIMS. It is necessary to irradiate a laser with a wavelength to be absorbed by a sample or a medium, an irradiation power density sufficient to vaporize sample molecules, and an appropriate pulse width (106 to 1010 W/cm2). A typical light source may be a Nd/YAG laser (wavelength: 266 nm, pulse width: 10 ns, pulse energy: 10 m) emitting fourth harmonics, which is used to realize a spot diameter of approximately 1 to 5 μm, in general.
MALDI is a method in which a laser beam is irradiated onto a surface of a sample to which a matrix that assists in ionizing organic molecules is added. This method has the advantage that the matrix suppresses decomposition of the organic molecules and accelerates desorption or ionization. In general, a light source may be a N2 laser (wavelength: 337 nm, pulse width: 4 ns), a Nd/YAG laser (wavelength: 355 nm, pulse width: 10 ns) emitting third harmonics, or the like with an irradiation power of approximately 105 to 108 W/cm2, which is considerably lower than that of LDI.
Although the use of SIMS achieves an excellent lateral resolution, it leads to the following problems. For example, organic molecules such as protein are destroyed due to an elastic collision between atoms in a biological sample and ions. As a result, measurement can be performed only once per unit, which is a very small division of a sample surface. Further, the production of secondary ions derived from organic molecules gradually is reduced to zero when the total irradiation amount of primary ions exceeds a certain value (static SIMS limit). The static SIMS limit of SIMS is about 1012×1013/cm2, and assuming that the primary ion current density is 1 nA/μm2, the irradiation time is about 15 to 150 μs, which becomes a big problem in imaging. As described above, when measurement can be performed only once and the production of secondary ions derived from organic molecules is low with poor ionization efficiency, sufficient measurement cannot be performed. In this manner, a method using SIMS has a problem in sensitivity. Further, SIMS also has a problem of charge-up of a sample due to the electric charge of the primary ions.
Further, since SIMS practically is intended only for a mass range of up to approximately 500, it is not suitable for the measurement of protein and the like. To solve this problem, liquid-SIMS (hereinafter, referred to as “LSIMS”) has been proposed, in which a nonvolatile liquid compound such as glycerol is added as a liquid matrix. With this method, the practical mass range can be expanded up to approximately 3000. However, although it is possible to expand the mass range and improve sensitivity by avoiding the problem involving the static SIMS limit, there is a problem in that a material distribution is disturbed.
On the other hand, the use of LDI does not have a problem of charge-up of a sample surface as in SIMS, and causes less decrease in the production of ions that occurs relative to the static SIMS limit of SIMS. However, only a very slight amount of ions can be produced per pulse, and thus it is required to perform measurement and signal integration repeatedly by performing pulse irradiation a plurality of times. Accordingly, this method also has a problem in sensitivity.
As compared with SIMS and LDI practically intended only for a mass range of up to approximately 500, MALDI enables the measurement of a target such as protein whose mass range is beyond the above-described range with very excellent sensitivity. However, there is a problem in that a material distribution may vary depending on the matrix composition and a method of adding the same. Further, due to energy propagation in a matrix, a region where ions are produced becomes larger than the diameter of an irradiation spot. As a result, it is difficult to achieve a high lateral resolution even by converging a laser beam to the maximum extent possible.    Non-Patent Document 1: Yasuhide NAITO, “Mass Microprobe Aimed at Biological Samples”, J. Mass. Spectrom. Soc. Jpn. Vol. 53, No. 3, pp. 125-132, 2005    Non-Patent Document 2: Shuichi SHIMMA, Mitsutoshi SETOU, “Review of Imaging Mass Spectrometry”, J. Mass. Spectrom. Soc. Jpn. Vol. 53, No. 4, pp. 230-238, 2005